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Development of provisional guidelines for the treatment of scale and resolution in assessing streamflow reduction impacts of alien plant infestations and commercial afforestation in water resources modelling studies

Dzvukamanja, T. N. (Tembiwe Nunana) (2005-12)

Thesis (MScEng)--University of Stellenbosch, 2005.

Thesis

ENGLISH ABSTRACT: Experiments conducted on afforested catchments in South Africa have shown that alien trees
can cause substantial reductions in catchment runoff (Scott et ai, 2000).
In recognition of the impact which alien trees can have on the country's water resources,
commercial afforestation was declared a stream flow reduction activity (SFRA) in terms of
the National Water Act (NW A) (No. 36 of 1998), and the Department of Water affairs and
forestry launched the Working for Water Programme (WfW) in 1995 with the recovery of
water resources lost to Invasive alien plants (lAPs) as one of the Programme's objectives.
These initiatives have intensified the need to quantify SFR; for example, for licensing
purposes to satisfy the requirements of the NWA and for predicting the effects of lAP clearing
by WfW projects. Of interest to water resources practitioners, is the impact of SFR on mean
annual runoff (MAR), on low flows and on water resource system, or reservoir, yield.
In South Africa two basic methods of streamflow reduction (SFR) estimation have been
developed for commercial afforestation and lAPs. These are
• free-standing empirical relationships in the form of the CSIR SFR CUNes, used in
conjunction with the monthly, calibration-based, Pitman model.
• component modules in the physically-based, land-use sensitive ACRU rainfall-runoff
catchment model, run at a daily time step with relatively fine subcatchment delineation.
There has been a strong need for an evaluative comparison of the impacts of SFR estimated
via these two methods. This study aimed to meet this need by using both methods to
estimate SFR for a number of commercial afforestation and lAP scenarios in three study
systems, the Berg, Sabie and Mhlatuze, representing different bioclimatic conditions in South
Africa, and running the SFR sequences from the two estimation methods through the Water
Resources Yield Model to determine the impact of the SFR on yield. The analysis
differentiated between upland and riparian SFR, and between SFR produced by different tree
classes.
Study conclusions included the following points:
• Both the ACRU and SHELL models are capable of achieving a reasonable average
seasonal correspondence of high and low flows with the observed averages, though the
actual averages produced by the two models can differ substantially. In general, ACRU simulates less SFR than SHELL, and gains in SFR after afforestation
or invasion by lAPs may be simulated by ACRU during dry periods. The selection of crop
factors for different plant species has a strong influence on the relative water use of the
species modelled in ACRU.
• The impacts on yield of SFR due to lAPS and afforestation tends to be greater than the
impact on MAR, and impacts tend to be more severe for small subcatchments than for
the total catchment. A simulated reduction in MAR can result in a simulated increase in
yield of a given assurance, if the portion of the flow sequence occurring during the critical
period is dominated by streamflow gains, and vice versa.
Research recommendations centred on improving the availability of reliable field
measurements of parameters and processes required tor the effective modelling of SFR.
Based on the results of the study, guidelines were formulated for SFR modelling, focussing
on the choice of SFR estimation method and the treatment of various parameters and
considerations which influence the outcomes of SFR modelling.